Waste-heat recovery systems comprising a waste-heat recovery circuit can utilize for example the waste heat in an internal combustion engine in a motor vehicle. For this purpose, said waste heat is applied to a steam generator. The working fluid circulating in the steam circuit process is thereby heated, evaporated and overheated. The hot working fluid, which is under high pressure, is then expanded in an expansion machine and performs mechanical work, which can be used for instance as additional vehicle drive or to drive a generator or an air conditioning system.
The steam generator is typically formed by a heat exchanger, through which a working fluid can be guided to absorb heat.
In the expansion machine, for example an axial piston machine, the working fluid is expanded from the high first pressure level to a lower second pressure level by performing work. The pistons thereby drive a shaft, which serves for example to move a vehicle. The expanded fluid is cooled and liquefied in a condenser and is supplied to the fluid circuit again via a pump. The higher the pressure and temperature difference, the higher the efficiency of the unit.
Water can be used as working fluid, the steam of which is relaxed by outputting work. For example, organic working fluids or water comprising additives can be used as well, which may be valuable or harmful to the environment. An escape of the working fluid is then unwanted. A condenser arrangement located in the waste-heat recovery system downstream of the expansion machine serves to liquefy the expanded working fluid. Typical temperatures of the working fluid are several hundred ° C. for the energy-rich steam state and in the case of water 100° C. as condensation temperature. The condensed working fluid is supplied to a working fluid reservoir, typically in the form of a suitably realized container, which is present in the waste-heat recovery circuit, where it is available again for the waste-heat recovery circuit without losses.
In this context, DE 10226445 C1 as well as WO 2005/001248 A1 disclose conventional waste-heat recovery circuits. Feed water is used as working fluid. The water is evaporated in an evaporator. The steam is expanded by performing work in an expansion machine. After the expansion, the steam is condensed in a condenser and is supplied by means of an electrically or mechanically operated pump to a reservoir, from which it is available for the circuit again. The described working machine is used for example as auxiliary device in motor vehicles. To also use it in the winter, it is known to add antifreeze to the working fluid. It is further known to add lubricant, for instance oil, to the working fluid. Depending on the temperature in the steam generator, organic working fluids, comprising a lower boiling point and/or combustible working fluids are used as well.
The described arrangements can overheat, when for example too much heat is supplied to the steam circuit process. The components of the waste-heat recovery circuit may then be damaged.